Nickel or cobalt base with a coating containing iron chromium and aluminum



3,542,530 NICKEL R COBALT BASE WITH A COAT- IN G CONTAINING IRON CHROMIUM AND ALUMINUM Frank P. Talboom, Jr., Glastonbury, and Johannes Grafwallner, South Glastonbury, Conn, assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware No Drawing. Filed May 23, 1968, Ser. No. 731,650 Int. Cl. B32b 15/00 US. Cl. 29183.5 4 Claims ABSTRACT OF THE DISCLOSURE A coating composition consisting of iron, chromium, aluminum and yttrium is provided to impart superior hightemperature oxidation, sulfidation, erosion and thermal Sl'l10Ck resistance to the nickel-base and cobalt-base supera loys.

CROSS-REFERENCE TO RELATED APPLICATION This application is related to the copending application entitled Method for Coating the Superalloys, Ser. No. 731,649, by R. C. Elam, J. O. Petrusha and F. P. Talboom, Jr.

BACKGROUND OF THE INVENTION The present invention relates to coated articles and coating compositions therefor and, more particularly, to a coating composition of iron, chromium, aluminum and yttrium having particular utility in imparting high temperature oxidation resistance to the nickel-base and cobalt-base superalloys.

As described in the patent to Hoyt et al. 1,995,923, alloys in the iron-chromium-aluminum system have been available for a number of years, particularly for use in high-temperature heating elements. More recently, it has been found that addition of small amounts of yttrium, together with the aluminum, will provide superior oxidation resistance, improved workability and better oxide films to the iron-chromium alloys. In the patent to McGurty et al. 3,027,252, for example, there is described an alloy consisting of, by weight, 20-95 percent chromium, 0.5-4 percent aluminum, 0.53 percent yttrium, balance iron. The patent to Wukusick, 3,298,826, discloses a somewhat similar alloy consisting of, by weight, Ol5 percent chromium, 05-12 percent aluminum, 0.1-3 percent yttrium, balance iron.

Basically, the prior art has been concerned with providing a good high temperature alloy with not only good oxidation resistance but also good structural strength and workability, and the chemistry of the various related alloys was formulated on this basis. The criteria for a coating composition, however, are apt to be different from those applicable to structural alloys. This has been borne out by tests establishing that the prior art formulations are unsatisfactory in providing long term surface protection to the nickel-base and cobalt-base superalloys when used as coatings therefor at the temperatures of interest in the current gas turbine engines.

SUMMARY OF THE INVENTION Briefly stated, the present invention contemplates a coating composition comprising, by weight, about 20-50 percent chromium, -20 percent aluminum, 0.03-2 percent selected from the group consisting of yttrium and the rare earth elements, balance iron, and articles coated with this composition.

In its most preferred embodiment, the coating of this invention is formulated to a composition comprising, by

rates- Patent Oice Patented Nov. 24, 1970 weight, 2529 percent chromium, 12-14 percent aluminum, 0.60.9 percent yttrium, balance iron, as applied to the nickel-base and cobalt-base superalloys.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the preferred coating is applicable to the nickelbase and cobalt-base alloys generally, it was formulated to the composition having particularly efficacious results with the nickel-base and cobalt-base superalloys. The superalloys will be understood to be those strong, hightemperature materials which find particular utility in the very demanding environments such as gas turbine engines. Representative of these superalloys are those identified in the industry as follows:

Alloy: Composition (percent by weight) IN 10 Cr, 15 Co, 4.5 Ti, 5.5 A1, 3 Mo, .17 C, .75 V, .075 Zr, .015

B, balance Ni. MAR-M200 9' Cr, 10 Co, 2 Ti, 5 A1, 12.5 W, .15 C, 1 Nb, .05 Zr, .015 B,

balance Ni. WI 52 21 Cr, 1.75 Fe, 11W, 2(Nb+Ta),

.45 C, balance Co. MAR-M302 21.5 Cr, 1 Fe, 10 W, 9Ta, .85 C,

.25 Zr, balance Co.

The characteristic of the typical superalloy is its basis as a nickel-chromium or cobalt-chromium solid solution with the additions usually aluminium, titanium and/or refractory metals for solution strengthening, and carbon, boron and zirconium to promote creep-rupture ductility. Taken as a class, the superalloys exhibit relatively good oxidation resistance at the temperatures associated with the hot section of a jet engine. However, since a compromise has normally been made in the alloy composition to achieve the best balance between strength and oxidation resistance as well as other factors, it is the usual practice to coat certain of the components formed from these superalloys to improve their oxidation, sulfidation, erosion and thermal shock resistance, and thus to extend their operating lives.

The Fe-CnAl-Y alloys, as formulated to the prior art chemistry, when applied as a coating to superalloy turbine components, have proved unsatisfactory. While the short term performance of the coating is good, there is a progressive deterioration thereof with time at elevated temperature.

It has now been discovered that an alloy of the composition, by weight, 20-50 percent chromium, 10-20 percent aluminum, 0.03-2 percent yttrium or a rare earth, such as lanthanum or scandium, balance essentially iron, will not only maintain the short term oxidation resistance but will provide long duration protection as well to those engine components to which it is applied. Studies revealed that the conventional alloys when utilized as coatings, rapidly diffused into the base metal depleting the aluminum level in the coating to a point Whereat the protective oxide layer could not be reestablished as the oxide was eroded away in the high velocity gas flow. Usually no stable oxide is formed at coating compositions below about 3-8 weight percent aluminum. Only when the aluminum content of the coating was raised to a minimum of about 10 percent was satisfactory long term coating performance attained. Coincidently, improved coating performance was attained with relatively high chromium contents. In general, the minimum quantities of the elements chromium, aluminum, and yttrium or rare earth are necessary to establish the desired oxidation resistance in the coating while the maximum quantities are an incident of the physical properties such as strength, melting point and oxide adherence.

The yttrium rare earth addition appears to lend stability to the protective oxide. As the best balance between components in the coating composition, the formulation, 25- 29 weight percent chromium, 12-14 weight percent aluminum, 0.6-0.9 percent yttrium, balance iron, is preferred.

In coating the nickel-base and cobalt-base turbine blades and vanes, the surfaces to be coated are first thoroughly cleaned free of all dirt, grease and other objectionable foreign matter followed by conditioning by means of abrasive blasting. The coating is achieved by vapor deposition from a molten pool of the coating material held in a vacuum chamber at torr or better. The melt chemistry in the preferred process is of the following composition:

Carbon max .02 Chromium 26-28 Aluminum 12.5-13.5 Yttrium 0.65-0.75 Phosphorus max .01 Sulfur max .01 Oxygen max .01 Nitrogen max .005 Hydrogen max .005 Other elements, total max 0.5 Iron Remainder Parts are preheated to 1750 F.:50 for 5-6 minutes before deposition is initiated and this temperature is maintained throughout the coating operation. Deposition time varies somewhat but is controlled to obtain the preferred coating thickness of .003-.005 inch. Subsequent cooling to below 1000 F. is accomplished in a non-oxidizing atmosphere at a rate equivalent to air cooling. Following the coating step, the parts are heat treated for 4 hours at 1900 F.i25 in vacuum.

The articles are then dry glass bead peened using .007- 011 inch diameter beads with an intensity equivalent to N. Further details relative to the significance of this working process are set forth in the copending application entitled Method for Coating the Superalloys, Ser. No. 731,649, previously mentioned. In general, the peening is conducted in accordance with the previsions of the processing specification AMS 2430. The parts are then heated to 1975 F.i in dry argon, dry hydrogen or vacuum; held at heat for 4 hours; and cooled in the protective atmosphere at a rate equivalent to air cooling.

The blades and vanes so processed exhibit a coating thickness, excluding diffused zone, of .O03-.005 inch. The diffused zone for the nickel alloys is .001-.002 inch and for the cobalt alloys .0005-.0015 inch.

While the present invention has been described in connection with certain preferred embodiments, these will be understood to be illustrative only. Those modifications to the invention evident to those skilled in the art from the teachings herein will, in the true spirit of the invention, be embraced wthin the scope of the appended claims.

What is claimed is:

1. A composite comprising:

a substrate formed from a nickel-base or cobalt-base superalloy; and

a coating thereon which consists essentially of, by

weight, 20-50 percent chromium, 10-20 percent aluminum, 0.02-2 percent selected from the group consisting of yttrium and the rare earth elements, balance iron.

2. A composite comprising:

a substance formed from a nickel-base or cobalt-base superalloy; and

a coating thereon which consists essentially of, by

weight, 20-50 percent chromium, 10-20 percent aluminum, 0.03-2 percent yttrium, balance iron.

3. A composite for gas turbine engine use comprising:

a substrate formed from a nickel-base or cobalt-base superalloy; and

a coating thereon which consists essentially of, by

weight, 25-29 percent chromium, 12-14 percent aluminum, 0.6-0.9 percent yttrium, balance iron.

4. A composite according to claim 3 wherein:

the coating thickness is 0003-0005 inch.

References Cited UNITED STATES PATENTS 2,105,293 1/1938 Godecke -126 2,946,676 7/1960 Brennan 75124 2,955,937 10/1960 McGutry 75-176 3,027,252 3/1962 McGurty 75-124 3,113,991 12/1963 Kleber 75126 3,298,826 1/1967 Wukusick.

HYLAND BIZOT, Primary Examiner US. Cl. X.R.

222 3? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,542,530 Dated November 24, 1970 Frank Pv Talboom, Jr. et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[- Column 4, line 16 002 should appear as 0.03.

Column 4, line 20 substance should appear as substrate.

TIM-:15 Am; main Tr E 197! FEB. 9,1971

4m mm W J mm 1:. J I I. omission of Patent 

